In the prior art, medical imaging system, in particular x-ray systems, are known, which are characterized by extensive flexibility in the movements of the image recording devices about the patients. In the case of an x-ray system, these moveable parts are in particular the x-ray detector and the x-ray tube. The so-called C-arm x-ray systems, in which the x-ray tube and detector are each affixed to arms of a C-arm opposing one another, are particularly popular, said C-arm being moveable in any manner about the patient in order to enable x-ray recordings from any projection directions. By varying the distance between the x-ray detector and the patient, the magnification and the image-distorting scattered radiation can furthermore be minimized. Such systems, such as for instance the AXIOM Artis by Siemens AG, are used in particular as angiography systems. Quasi-tomographic 3D images are increasingly generated using such x-ray devices, in which the C-arm is moved about the patient at approximately 180°. With such a run, which is also referred to as “Dynarun”, the moveable parts are partly rotated about the patient at a considerable speed. One problem with this type of imaging system with moveable parts thus consists in the possibility of a collision of the moveable parts with the patient. To rule out dangers to the patient, protective mechanisms must thus be used.
In the meantime, digital imaging methods play a decisive role within medical diagnostics. Whilst digital techniques were used from the start in diagnostics methods, such as for instance computed tomography, magnetic resonance, ultrasound and nuclear medicine methods, the transition to digital imaging is currently taking place to a great extent, particularly with the aid of surface detectors in “conventional” x-ray methods, such as for instance radiography, mammography, angiography or also cardiology (see Spahn et al. “Digitale Röntgen-Detektoren in der Röntgendiagnostik”, [Digital x-ray detectors within x-ray diagnostics], Radiologie 43 (2003), pages 340 to 350 for instance).
These new methods require ever faster acquisitions and/or an ever larger number of images for a three-dimensional image acquisition, with robot arms being used in the meantime in the case of conventional projection radiography for instance, said robot arms being able to be controlled by selecting an organ program and being automatically moved into a new position. It is of interest here to implement the method as quickly as possible, in order to speed up hospital procedures.
The risk here is the possibility of a collision of the moveable part, such as for instance the robot arm, with the patient. The risk of a collision between this support arm and the patient or the support arm and another mechanical component such as the table for instance and a second support arm or other medical devices particularly exists with fast movements of mechanical support arms. Different devices and methods are known here to avoid a collision or to lessen the effect of a collision.
With the above-mentioned system AXIOM Artis, a protective zone encasing the patient is defined for instance. If a C-arm approaches this protective zone, it is considerably slowed down in order to avoid the risk of a collision. This protective zone is the same for all patients and takes the shape of an approximate ellipsoid arranged over the patient support. With patients of low weight in particular, this protective zone is thus often remote from the actual patient surface. It thus often takes an unnecessarily long time to run a specific angulation of the C-arm.
With devices made by the company Philips, capacitative sensors are partly arranged on the C-arm, said sensors detecting the proximity of the patient and thereupon slowing down the movement of the C-arm. These sensors nevertheless only have a minimal coverage, so that the intention here is to slow down the movement of the C-arm when entering an accepted protective zone, as rapid movements can otherwise result in a collision.
Both systems feature mechanical position indicators as a final safety device, which in the event of actual contact with the patient, immediately stop the movement of the moveable parts (cf. also EP 0 395 352 A1, which discloses a decoupling of the arm engine from the power supply in the event of a collision).
One disadvantage with the mechanical design of an anti-collision method or the calculation of a protective zone is that the method is either based on contact or requires significant computing effort, so that with increasingly faster movements of the moveable part, the response time, which is hereby ever shorter, is often insufficient to actually prevent a collision, and/or increasingly higher braking pulses are needed to intercept the moveable part. This in turn requires steadier and more powerful motors and/or braking systems, which in turn increases the costs and the dimensions or the systems, whereby the braking pulses and the course of motion of the moveable part is finally once again negatively influenced.